The present invention relates to fuser apparatuses and fusing members thereof, in electrostatographic reproducing, including digital and image-on-image, apparatuses. The fuser member is especially useful for fusing color images. More specifically, the present invention relates to apparatuses directed towards fusing toner images using an external fusing member to aid in maintaining sufficient heat to the fuser member. In preferred embodiments, the external fusing member has an outer fluoropolymer layer. In a particularly preferred embodiment, the fluoropolymer outer layer is filled with a conductive filler, preferably silicon carbide. In a particularly preferred embodiment, the to silicon carbide filler has a very fine particle size of less than about 10 microns.
In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well known. To fuse electroscopic toner material onto a support surface permanently by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes it to be firmly bonded to the support.
Several approaches to thermal fusing of electroscopic toner images have been described. These methods include providing the application of heat and pressure substantially concurrently by various means, a roll pair maintained in pressure contact, a belt member in pressure contact with a roll, a belt member in pressure contact with a heater, and the like. Heat may be applied by heating one or both of the rolls, plate members, or belt members. Heat may be applied to the fuser members by internal and/or external sources.
In color copying and printing, normally customer preference for color prints is a high gloss or matte opaque finish. This usually requires the use of smooth, conformable fuser roll operating at a high temperature and having a long-dwell nip. In addition, extra release agent is necessary for improving toner release due to the increase in toner used for color developing. For developing color images, several layers of different color toner are deposited on the latent image resulting in extra thickness (higher toner pile height) of unfused toner on a color image. Therefore, a higher operating temperature for color fusers is necessary to fuse the additional amount of toner. Also, as the need for increased speed and production in copying and printing occurs, it is desired that the fusing temperature remains elevated for longer time periods.
If the temperature of the fusing member is increased to the point necessary for color fusing, a problem frequently encountered is that the copy substrate, e.g. a sheet of paper, on which the toner image is fused, may curl and/or adhere to the heated fuser. Such adhering paper will tend to wrap itself around the fuser and thus prevent the fuser from performing its intended operations in subsequent copying cycles. Such adhering paper must be generally removed by hand, resulting in manual labor, machine downtime, and customer dissatisfaction.
Another feature common to most of the prior art fusing members is that the source of the heat energy for the fusing operation is generally in the form of a quartz lamp positioned in the core of a fuser roll. In such a configuration, the heat must be conducted from the core of the fuser member, through the various layers of materials comprising the fuser member, to the surface of the fuser member for fusing the toner image to the copy substrate. To obtain the proper higher fusing temperature needed for color fusing at the surface of such a fusing member, the temperatures at the various layers or points within the fuser member must be substantially higher. Since heat must be transmitted from the source in the core of the fuser member to its surface, it takes an appreciable amount of time before the surface of the fusing member is warmed up to the fusing temperature and thus ready for operations. This delay in readiness of the machine to fuse toner images, or the warm-up time, is accentuated by the fact that such fuser members are generally made of elastomeric or other polymeric materials which are generally poor conductors of heat.
To solve some of the above problems that occur with fuser members which require heating to such higher temperatures necessary in color fusing, an external heat member has been used. This external heat member is associated with the fusing member so as to provide additional heat to the surface of the fusing member to account for the additional surface heat necessary for color fusing.
U.S. Pat. No. 3,452,181 discloses a roll fusing device which is heated by both an internal heating element and an external auxiliary heating element. The fusing drum of this patent is made of a glass or quartz sleeve having a transparent silicon varnish layer thereon and offset-preventing silicone oil is applied to the surface of the silicone varnish layer.
U.S. Pat. No. 4,071,735 discloses an externally heated roll fuser, in which the heating element heats the fuser roll at the same time preheats the toner image to be fused. The fuser roll of this patent is made of a metallic core with a layer of heat insulating silicone rubber thereon.
U.S. Pat. No. 4,372,246 teaches an externally heated fuser member which comprises a base, a relatively thick layer of a foam of fluoroelastomer on the base and a relatively thin layer of a silicone elastomer on the foam layer. The silicone elastomer layer has an iron oxide filler dispersed therein.
U.S. Pat. No. 5,349,424 discloses a thick walled belt fusing system having an externally heated fuser roll associated therewith, for use in full color electrophotographic printing machines.
U.S. Pat. No. 5,291,257 discloses a composite pressure roll having a surface coating of a fluorocarbon polymer and an irregularly shaped, non-planar, inert filler having a hardness greater than 8 Mohs, and having a particle size of from about 10 to about 30 microns and present in the coating in an amount of from about 10 to about 40 percent by weight of total solids.
Although external heat members provide benefits to color fusing, such as increasing the temperature of the fuser member necessary for color fusing, problems with use of external heat members have arisen. For example, although the external heat roll increases heat to the surface of a fuser member, the heat transfer has been found to interfere with the release properties of the surface of the fuser member. Specifically, toner remaining on the fuser member following fusing can be transferred to the external heat member, and retransferred to the fusing member upon the next fusing cycle. Further, as the desire for faster copiers and printers increases, faster output is required and higher heat is required for the fusing system to maintain the increased speed. Further, sufficient heat at a required relatively high temperature must be maintained for longer periods of time. Even with the help of an external heating member, the temperature tends to decrease the longer the fuser member is in use. This is known as temperature droop.
It is desired to provide an external fuser member, wherein high quality color prints and/or copies are produced. Particularly, it is desired to provide an external fuser member demonstrating increased thermal conductivity and improved temperature control. More specifically, an external heat member which increases the temperature of the fusing member to the relatively high temperature necessary in color fusing, and which maintains the fuser member at that temperature for longer periods of time is desired. Further, it is desired to provide an external heat member which decreases the contamination to the fusing member.